Method and device for obtaining pure nitrogen and pure oxygen by low-temperature separation of air

ABSTRACT

Feed air is compressed to a first pressure in a main air compressor. A first sub-stream of the air compressed to the first pressure is cooled and fed at least in part to the distillation column system. A second sub-stream of the air compressed to the first pressure is cooled and at least partially liquefied in a low-pressure column bottom evaporator. The at least partially liquefied second sub-stream is introduced at least in part into the distillation column system. A liquid oxygen-enriched fraction is introduced into the evaporation chamber of a high-pressure top condenser. An argon-containing oxygen stream from an intermediate point in the low-pressure column is introduced into an argon column. The second sub-stream is introduced at least in part into an argon top condenser and partially evaporated therein. The second sub-stream is then introduced at least in part into the high-pressure column and/or into the low-pressure column.

The invention relates to a process for producing pure nitrogen and oxygen by cryogenic fractionation of air according to the preamble of claim 1.

Processes and apparatuses for cryogenic fractionation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik [Cryogenics], 2nd edition 1985, chapter 4 (pages 281 to 337).

The distillation column system may take the form of a two-column system (for example of a conventional Linde twin-column system), or else of a three- or multicolumn system. In addition to the columns for nitrogen-oxygen separation, it may have further apparatuses for obtaining high-purity products and/or other air components, especially noble gases, for example argon production and/or krypton-xenon production.

An “argon discharge column” is understood here to mean here to a separating column for argon-oxygen separation which does not serve to produce a pure argon product, but serves to discharge argon from the air which is being fractionated in the high-pressure column and low-pressure column. It is connected in a manner only slightly different than that of a conventional crude argon column which generally has 70 to 180 theoretical plates; however, it contains distinctly fewer theoretical plates, namely fewer than 40, especially between 15 and 30. As in the case of a crude argon column, the bottom region of an argon discharge column is connected to an intermediate point in the low-pressure column and the argon discharge column is cooled by a top condenser, with introduction of expanded bottoms liquid from the high-pressure column or a similar coolant on the evaporation side thereof; an argon discharge column generally does not include a reboiler.

The word “argon column” is used here as an umbrella term for argon discharge columns, full-scope crude argon columns and all intermediate stages in between.

A “main heat exchanger” serves to cool feed air in indirect heat exchange with return streams from the distillation column system. It may be formed of a single heat exchanger section or a plurality of operatively connected, parallel- and/or series-connected heat exchanger sections, for example of one or more plate heat exchanger blocks.

The expression “condenser-evaporator” refers to a heat exchanger in which a first, condensing fluid stream enters into indirect heat exchange with a second, evaporating fluid stream. Every condenser-evaporator has a liquefaction space and an evaporation space, which consist of liquefaction passages and evaporation passages respectively. The condensation (liquefaction) of the first fluid stream is conducted in the liquefaction space, the evaporation of the second fluid stream in the evaporation space. The evaporation space and the liquefaction space are formed by groups of passages which are in a heat-exchanging interrelationship. The evaporation space of a condenser-evaporator may take the form of a bath evaporator, a falling-film evaporator or a forced-flow evaporator.

A relevant process is known from Petras/Mostello, Experience with supplying oxygen to an IGCC power plant and evaluation of alternative supply arrangements for future coal gasification facilities, 6th EPRI Coal Gasification Contractors Conference, October 1986, FIG. 6-6. It serves to produce 96% oxygen as the main product.

A process of the type specified at the outset and a corresponding apparatus are known from U.S. Pat. No. 4,854,954.

It is an object of the invention, in a process of this kind, to produce both oxygen and nitrogen in high purity, i.e. with a purity of at least 99 mol %, preferably more than 99.99 mol %, in the case of nitrogen and with a purity of at least 96 mol %, preferably more than 99.5 mol %, in the case of oxygen.

This object is achieved by the characterizing features of claim 1.

Argon columns are generally cooled with oxygen-enriched liquid from the high-pressure column (see, for example, FIGS. 6-9, 6-11 and 6-12 in the abovementioned article by Petras/Mostello). Systems according to the preamble in which the cooling capacity of the oxygen-enriched liquid from the high-pressure column is already being used to cool the tops from the high-pressure column have therefore never to date been provided with an argon column. In the context of the invention, it has been found that, surprisingly, it is possible to condense sufficient air in the low-pressure column reboiler that it is sufficient as a coolant for the top condenser of an argon column. In this way, in the context of the invention, the condenser configuration of high-pressure column and low-pressure column according to the preamble of claim 1 can advantageously be combined with an argon column. In this way, the argon present in the feed air can be discharged and optionally obtained as a product. Through the discharge of the argon, in the invention, it is possible to produce both the nitrogen and the oxygen as pure products.

It is advantageous when the first substream of the feed air, upstream of its introduction into the high-pressure column or into the low-pressure column, is expanded to perform work in a first expansion machine. For this purpose, a moderate-pressure turbine or an air injection turbine is used. In a specific configuration, it is also possible for both a moderate-pressure turbine and an air injection turbine to be provided, which are then operated with different portions of the feed air (two-turbine process).

Preferably, the first substream, upstream of its expansion to perform work, is recompressed to a second pressure higher than the first pressure. For this purpose, it is possible to use an external-energy-driven recompressor and/or a turbine-driven recompressor.

The process of the invention can also be operated with internal compression. In internal compression, a product stream pressurized in liquid form is evaporated against a heat carrier and ultimately obtained as a gaseous compressed product. In the case of a supercritical pressure, no phase transition per se takes place; the product stream is then “pseudo-evaporated”. In this configuration of the invention, a liquid fraction from the distillation column system in the liquid state is brought to an elevated product pressure, warmed in the main heat exchanger at this elevated product pressure and finally drawn off as a gaseous compressed product; a third substream of the feed air compressed to the first pressure as heat carrier is recompressed to a third pressure higher than the first pressure and especially higher than the second pressure, and then cooled in the main heat exchanger; the cooled third substream is expanded and introduced into the high-pressure column and/or the low-pressure column. The third substream is liquefied in the main heat exchanger (or—in the case of supercritical pressure—pseudo-liquefied) and enters the separation column(s) essentially in the liquid state.

The liquid fraction from which the internally compressed product is obtained can, according to claim 5, be formed either by oxygen from the low-pressure column or by nitrogen from the high-pressure column; in addition, it is possible to produce both fractions simultaneously as internally compressed products.

The (pseudo-)liquefied third substream of the feed air from the main heat exchanger is preferably expanded to perform work in a dense liquid expander before it is fed into the separation column(s).

As already mentioned, in addition to a moderate-pressure turbine, it is possible to expand a fourth substream of the feed air to perform work in a second expansion machine which takes the form of an air injection turbine, and then introduce it into the low-pressure column. In this way, it is possible to remove a relatively high proportion of the products in liquid form.

The liquid oxygen-enriched fraction which is introduced into the evaporation space of the high-pressure column top condenser may in principle originate from an intermediate point in the high-pressure column. In a first variant, it is formed by at least a portion of the bottoms liquid from the high-pressure column. In this case, the high-pressure column top condenser is preferably disposed at the top of the high-pressure column.

In a second, differing variant, the liquid oxygen-enriched fraction for the high-pressure column top condenser is formed by an intermediate liquid from the low-pressure column. In this case, the high-pressure column top condenser may be disposed within the low-pressure column, separately from the low-pressure column and high-pressure column or at the top of the high-pressure column.

The invention also relates to an apparatus as claimed in claim 15. The apparatus of the invention may be supplemented by apparatus features corresponding to the features of individual, multiple or all dependent process claims.

The invention and further details of the invention are elucidated in detail hereinafter with reference to working examples shown in schematic form in the drawings. The figures show:

FIG. 1 a first working example of the invention, in the form of a two-turbine process,

FIG. 2 a second working example of the invention, in the form of a one-turbine process,

FIG. 3 a further working example of the invention in which the high-pressure column top condenser is incorporated into the low-pressure column,

FIG. 4 a system of the invention with an additional intermediate evaporator in the low-pressure column,

FIG. 5 a working example of the invention in which the low-pressure column reboiler is executed as a dephlegmator,

FIG. 6 a different turbine configuration of the invention with two turbines in series and intermediate cooling,

FIG. 7 another turbine configuration of the invention with two turbines in series and intermediate warming,

FIG. 8 a specific machine configuration of the invention in which both turbines and a single recompressor are disposed on a common shaft, and

FIG. 9 a working example of the invention with a specific arrangement of a liquid expander.

In the working example of FIG. 1, atmospheric air (AIR) is compressed in a main air compressor 1 with intermediate cooling and postcooling 2 to a first pressure of, for example, 4.3 to 5.0 bar, preferably 4.5 to 4.7 bar. The compressed air is cooled further in a precooling operation 3, for example a direct contact cooler, and sent to a cleaning apparatus 4 which is formed by a pair of molecular sieve adsorbers.

The cleaned feed air 5 at the first pressure, the example, is branched into a first substream 6, a second substream 7, a third substream 8 and a fourth substream 9. The first substream 6 and the third substream 8 are recompressed together (stream 10) in a first recompressor 11 with postcooler 12 to an intermediate pressure of, for example, 4.3 to 8.0 bar, preferably 4.5 to 6.0 bar, and branched off from one another at this intermediate pressure.

The first substream 6 at the intermediate pressure is compressed further in a second recompressor 13 with postcooler 14 to a second pressure of, for example, 4.5 to 9.0 bar, preferably 4.7 to 7.0 bar. The first substream 15 is then fed at the second pressure to the main heat exchanger 26 at the warm end, where it is cooled down to a first intermediate temperature. At the first intermediate temperature, the first substream 16 is fed to an expansion machine 17 in which it is expanded to perform work to about the operating pressure of a high-pressure column (see below). The expansion machine, like all the expansion machines in the working examples, is executed as a turboexpander. The expansion machine 17 is also referred to as moderate-pressure turbine and drives the second recompressor 13 by means of a common shaft. The first substream 18 that has been expanded to perform work is fed via conduit 19 to the high-pressure column 20, immediately above the bottom.

The high-pressure column 20 is part of a distillation column system which also has a low-pressure column 21, an argon column 22 and the accompanying condenser-evaporator, namely a high-pressure column top condenser 23, a low-pressure column reboiler 24 and an argon top condenser 25.

The second substream at the first pressure is cooled in the main heat exchanger 26 and optionally partly liquefied, and withdrawn at the cold end of the main heat exchanger 26. The vapor component in the cooled second substream 30 is more than 70 mol %, preferably more than 98 mol %. The cooled and mainly gaseous second substream 30 is introduced into the liquefaction space of the low-pressure column reboiler 24. It flows through the liquefaction space therein, preferably from the top downward, and is at least partly, preferably completely or virtually completely, liquefied in indirect heat exchange with the partly evaporating bottoms liquid from the low-pressure column 21. The vapor component on exit from the low-pressure column reboiler 24 is less than 10 mol %; the second substream at this point is preferably completely liquid, but not subcooled. The liquid second substream 31 is then passed into the evaporation space of the argon top condenser 25 and expanded to the suitable pressure beforehand in an expansion apparatus 32 (for example a valve). The fraction 33 of the second substream evaporated in the condenser 25 is fed into the high-pressure column 20 via conduit 19. The remaining liquid 86 from the evaporation space of the argon top condenser 25 (which acts here as separation stage) is mixed with the bottoms liquid 62 from the high-pressure column 20.

The third substream 8 is compressed further from the intermediate pressure in a third recompressor 34 with postcooler 35 to a third pressure higher than the second pressure. The high-pressure air 36 is cooled and liquefied in the main heat exchanger 26 or (if the pressure is supercritical) pseudo-liquefied. The liquid or supercritical cold air is expanded to perform work in a dense liquid expander 38 to about 7.0 bar, in the lowest case to about high-pressure column pressure. Alternatively, the expansion of the liquid or supercritical cold air can take place in a Joule-Thomson valve in conduit 138. A first portion 40 of the expanded liquid air 39 is fed to the high-pressure column 20 at an intermediate point. A second portion 41 is cooled in a countercurrent subcooler 42 and, after appropriate expansion 43, fed into the low-pressure column 21.

The fourth substream 9 is compressed further in a third recompressor 44 with postcooler 45 from the first pressure to about 5.0 to 7.0 bar. The recompressed fourth substream 46 is fed to the main heat exchanger 26 at the warm end, where it is cooled to a second intermediate temperature which may be the same as or different than the first intermediate temperature. At the second intermediate temperature, the cooled fourth substream 47 is fed to an expansion machine 48 in which it is expanded to perform work to about the operating pressure of a high-pressure column (see below). This expansion machine too is executed as a turboexpander. It is also referred to as air injection turbine and drives the third recompressor 44 by means of a common shaft. The fourth substream 49 that has been expanded to perform work is fed to the low-pressure column 21 at an intermediate point. The second intermediate temperature is preferably chosen such that the thermodynamic state of the substream 49 is established not more than 10 K above its dew point and at a minimum of a 10% molar liquid fraction.

Gaseous tops nitrogen 50 is drawn off from the top of the high-pressure column 20 and a first portion 51 is essentially completely condensed in the high-pressure column top condenser 23. A first portion 53 of the liquid nitrogen 52 obtained is recirculated to the high-pressure column 20. A second portion 56 is used as reflux in the low-pressure column 21; a third portion 57 can be drawn off if required as liquid product (LIN). This nitrogen product has a purity of less than 1000 ppm of oxygen, preferably less than 10 ppm of oxygen. The second and third portions of the liquid nitrogen 52 are conducted together (54) to the countercurrent subcooler 42, where they are cooled, and then expanded to low-pressure column pressure in a valve 55. A fourth portion 58 is sent to an internal compression. In the liquid state, it is brought to an elevated first product pressure of typically more than 6 bar in a pump 59. The high-pressure nitrogen 60 at this elevated first product pressure is warmed in the main heat exchanger 26 and finally drawn off as gaseous compressed product (GAN-IC). A second portion of the gaseous tops nitrogen 50 from the high-pressure column 20 is warmed in the main heat exchanger 26 and obtained as gaseous moderate-pressure product (PGAN).

The liquid oxygen-enriched fraction 62 from the bottom of the high-pressure column 20 is conducted together with the liquid oxygen-enriched stream 86 from the evaporation space of the argon top condenser 25 via conduit 63 to the countercurrent subcooler 42, where it is cooled, throttled to about low-pressure column pressure in an expansion device 64 (for example a valve), and a first portion 65 is introduced into the evaporation space of the high-pressure column top condenser 23. The evaporation space of the high-pressure column top condenser 23 takes the form of a bath evaporator in the example. The remainder 66 flows via conduit 67 directly into the low-pressure column 21. The oxygen-enriched fraction 68 evaporated in the high-pressure column top condenser 23 is fed (69) to the low-pressure column 21. Purge liquid, i.e. unevaporated oxygen-enriched liquid 69, is introduced into the low-pressure column 21 at another point via conduit 67.

Via conduits 70 and 71 and the optional pump 72, the low-pressure column 21 communicates at an intermediate point with the bottoms from the argon column 22, which preferably takes the form here of an argon discharge column. More particularly, an argon-containing oxygen stream 70 is drawn off from an intermediate point in the low-pressure column 21 and introduced into the argon column 22. The tops gas 73 contains more than 50 mol %, preferably more than 70 mol %, of argon. It is introduced into the liquefaction space of the argon top condenser 25, where it is almost completely condensed. The condensate 74 is returned to the argon column 22 as reflux. The remaining gas (CGA=crude gaseous argon) is drawn off as a tail stream. It can subsequently either be discarded directly or after warming in the main heat exchanger 26 or be sent to a downstream separation step. The reflux liquid that arrives in the bottom of the argon column 22, optionally by means of the pump 72, is guided back into the low-pressure column 21 via conduit 71.

From the bottom of the low-pressure column 21, or more specifically from the evaporation space of the low-pressure column reboiler, liquid oxygen 76 is drawn off and a first portion 77 is obtained as liquid product (LOX) if required. The remainder 78 is sent to an internal compression. In the liquid state, it is brought to an elevated second product pressure of typically 6 to 30 bar in a pump 79. The liquid or supercritical high-pressure oxygen 80 at this elevated second product pressure is warmed in the main heat exchanger 26 and finally drawn off as gaseous compressed product (GOX-IC). The purity of the liquid oxygen from the low-pressure column bottom is more than 98 mol %, preferably more than 99.5 mol %.

Gaseous nitrogen 81 is drawn off from the top of the low-pressure column, warmed in the countercurrent subcooler 42 and further in the main heat exchanger 26, and obtained as uncompressed nitrogen product (GAN). Gaseous impure nitrogen 82 is drawn off from an intermediate point in the low-pressure column 21 and is likewise warmed in the countercurrent subcooler 42 and in the main heat exchanger 26. The warm impure nitrogen (UN2) 83 can be used partly 84 as regeneration gas for the cleaning apparatus 4, and partly 85 as dry gas for the precooling 3 in an evaporation cooler.

The relative height arrangement of high-pressure column 20, low-pressure column 21 and high-pressure column top condenser 23 is chosen such that the liquids produced in the condensers, especially the reflux liquids 53 and 56, reach their target with the aid of the natural gradients or pressure differentials, i.e. without a pump.

The operating pressures are:

-   -   2.5 to 3.5 bar, preferably 2.7 to 3.0 bar, at the lower end of         the high-pressure column 20,     -   1.3 to 1.5 bar at the lower end of the low-pressure column 21,     -   1.1 to 1.4 bar at the upper end of the argon column 22.

The high-pressure column contains 30 to 60, preferably 40 to 50, theoretical plates; these may be implemented, for example, by sieve trays or structured packing. The low-pressure column contains 90 to 160, preferably 120 to 150, theoretical plates; these may be implemented, for example, by sieve trays or structured packing. In the argon column there are 20 to 200 theoretical plates, preferably in the form of structured packing. In the working examples, the argon column takes the form of an argon discharge column, meaning that it serves solely or mainly to discharge argon and hence to increase the oxygen yield and/or the oxygen purity. In a different method, with a correspondingly high number of plates, the argon column may take the form of a true crude argon column that produces virtually oxygen-free crude argon and passes it on, for example, to a pure argon column for argon-nitrogen separation.

Another working example that is not shown in any drawing takes the form of a one-turbine process. This is apparent from FIG. 1 in that the fourth substream 9, 46, 47, 49, the turbine-recompressor combination 44/48 and the corresponding passage in the main heat exchanger 26 and conduit 66 are omitted. In this case, the oxygen-enriched stream 63 is introduced completely via conduit 65 into the top condenser 23, where it is partially evaporated. The gas phase 68 and the liquid phase 69 are fed to the low-pressure column, wherein the feed for the gas phase is above that for the liquid phase.

FIG. 2 also shows a one-turbine process. Here, the moderate-pressure turbine 17 from FIG. 1 has been omitted, but an air injection turbine 248 has been retained. In this case, the stream 209 forms the “first substream” and the air injection turbine 248 the “first expansion machine” according to claim 1. Otherwise, the description of FIG. 1 is analogously applicable to FIG. 2. In this example, the second substream 30 of the feed air is preferably completely liquefied in the low-pressure column reboiler 24. Between the feeds of streams 67 and 68 into the low-pressure column 21, there are 20 to 60 theoretical plates here. What is not shown in FIG. 2 is a withdrawal of liquid at the lowest point in the high-pressure column top condenser 23. Purging of the bath can be undertaken here.

FIG. 3 is based on the modified working example from FIG. 1 without an air injection turbine, i.e. is a one-turbine process with a moderate-pressure turbine 17. The bottoms liquid 62 from the high-pressure column 20 is not introduced here directly into the evaporation space of the high-pressure column top condenser 323 arranged as an intermediate evaporator within the low-pressure column 21. Instead, it is guided via conduit 367 to an upper intermediate point in the low-pressure column 21 and then flows together with the reflux liquid from the upper section of the low-pressure column 21 through at least one mass transfer section 387 before flowing into the evaporation space of the high-pressure column top condenser 323. The mass transfer section 387 can also be omitted.

FIG. 4, like FIG. 3, proceeds from the modified working example from FIG. 1 without an air injection turbine, i.e. is again a one-turbine process with a moderate-pressure turbine 17. Here, the condensation of the second substream 30 of the air is distributed between two condensers which are connected in series on the liquefaction side. Apart from the known low-pressure column reboiler 424, a low-pressure column intermediate evaporator 488 is additionally used here. The essentially gaseous second substream is at first only partly liquefied in the reboiler 424. The biphasic mixture 489—or after optional phase separation the gaseous component only—is then completely or almost completely liquefied in the intermediate evaporator 488 and then guided as usual through conduit 31 into the evaporation space of the argon top condenser 25. Between the two evaporators 424, 488, it is possible—but not obligatory—for an additional mass transfer section 487 to be incorporated. The air preferably flows in the direction of gravity through the reboiler 424 and the intermediate evaporator 488. Such an execution of the process enables a lowering of the exit pressure of the main air compressor of typically 30 to 100 mbar compared to the configurations of FIGS. 1 to 3.

In FIG. 5, a dephlegmator is used as low-pressure column bottoms heater 524. Otherwise, the working example of FIG. 5 again corresponds to FIG. 1 without an air injection turbine. On the evaporation side of the low-pressure column reboiler 524, countercurrent heat exchange takes place between gas and liquid within the heat exchanger passages. Evaporated oxygen can ascend upward and take part in mass transfer with the oxygen trickling downward in countercurrent by means of correspondingly configured internals.

In this case, the air preferably flows counter to gravity through the liquefaction passages. A greater portion of the separation performance of the low-pressure column than in the case of a bath evaporator is provided here by the evaporator. The second substream 30/31, by contrast, flows unchanged from the bottom upward through the liquefaction passages of the low-pressure column reboiler 524. Liquid and any remaining gas both leave the heat exchanger at the top end. Such an execution of the process enables a lowering of the exit pressure of the main air compressor of typically 50 to 150 mbar compared to the configurations of FIGS. 1 to 3.

FIG. 6 also dispenses with an air injection turbine and is otherwise largely identical to FIG. 1. The working example differs merely in the execution of the work-performing expansion of the first substream 15. The work-performing expansion from the second pressure to the high-pressure column pressure is conducted in two stages in two turbines 617 a, 617 b in series. The first, warm turbine 617 a provides expansion to an intermediate pressure. The resulting air stream 618 is cooled in the main heat exchanger 26 from a higher to a lower intermediate temperature. At the lower temperature, the second substream 616 enters the second, cold turbine, where it is expanded to about the operating pressure of the high-pressure column. The two recompressors 613 b, 613 a coupled to the turbines are connected in series. This arrangement allows further energy optimization of the process compared to the configurations of FIGS. 1 to 3.

FIG. 7 differs from FIG. 6 in that no intermediate cooling is conducted between the two turbines 617 a, 617 b; instead, intermediate warming is conducted. Stream 618 is warmed in the main heat exchanger 26 before being introduced via conduit 616 into the cold turbine 617 b. This arrangement allows further energy optimization of the process compared to the configurations of FIGS. 1 to 3.

FIG. 8 is identical to FIG. 6 for chemical engineering purposes. However, a combination machine by means of which the two turbines 617 a, 617 b are coupled to a single recompressor 813 is used.

FIG. 9 proceeds from the modified working example from FIG. 1 without an air injection turbine, i.e. is again a one-turbine process with a moderate-pressure turbine 17. Here, the liquid expander 938 is arranged downstream of the countercurrent subcooler 42. In this way, the pressure gradient that exists can be exhausted completely and, in addition, peak cooling can be generated.

In a further, modified working example, the second substream 37 can be expanded via a liquid expander 38 only; this is positioned between the main heat exchanger and countercurrent subcooler. In this case too, the pressure gradient that exists is exhausted completely since there is no longer any technically relevant lowering of the pressure via control element 43 from FIG. 1. 

1. A process for producing pure nitrogen and oxygen by cryogenic fractionation of air in a distillation column system having a high-pressure column (20) and a low-pressure column (21) and also a high-pressure column top condenser (23) and a low-pressure column reboiler (24, 424, 488, 524), both of which take the form of condenser-evaporators, wherein the entirety of the feed air is compressed to a first pressure in a main air compressor (1), a first substream (10, 6, 15, 16; 209, 246, 247) of the air compressed to the first pressure is cooled in a main heat exchanger (26) and at least partly fed (18; 249) to the distillation column system, a second substream (7) of the air compressed to the first pressure is cooled in the main heat exchanger (26) and then at least partly liquefied in the low-pressure column reboiler (24, 424, 488, 524), the at least partly liquefied second substream (31, 33, 19) is at least partly introduced into the distillation column system, a liquid, oxygen-enriched fraction (62, 86) is introduced (63, 64, 65) into the evaporation space of the high-pressure column top condenser (23), the distillation column system also has an argon column (22) with an argon top condenser (25) which takes the form of a condenser-evaporator, and an argon-containing oxygen stream (70) is drawn off from an intermediate point in the low-pressure column (21) and introduced into the argon column (22), characterized in that the second substream (31), downstream of the low-pressure column reboiler (24, 424, 488, 524), is at least partly introduced into the argon top condenser (25) where it is partly evaporated and the second substream (33), downstream of the argon top condenser (25), is at least partly introduced into the high-pressure column (20) and/or into the low-pressure column (21).
 2. The process as claimed in claim 1, characterized in that the first substream (16; 247), upstream of its introduction into the high-pressure column (20) or into the low-pressure column (21), is expanded to perform work in a first expansion machine (17; 248).
 3. The process as claimed in claim 2, characterized in that the first substream, upstream of its expansion to perform work, is recompressed (11, 13; 244) to a second pressure higher than the first pressure.
 4. The process as claimed in claim 1, characterized in that a liquid fraction (58; 78) from the distillation column system in the liquid state is brought (59, 79) to an elevated product pressure, warmed in the main heat exchanger (26) under this elevated product pressure and finally drawn off as a gaseous compressed product (GAN-IC; GOX-IC), a third substream (10, 8) of the feed air compressed to the first pressure is recompressed (11, 34) to a third pressure higher than the first pressure and especially higher than the second pressure, and is cooled in the main heat exchanger (26), and the cooled third substream (37) is expanded (38, 938) and introduced (40, 41, 43) into the high-pressure column (20) and/or the low-pressure column (21).
 5. The process as claimed in claim 1, characterized in that the liquid fraction is formed by a liquid oxygen stream (78) from the low-pressure column (21) or from the low-pressure column reboiler (24, 424, 488, 524), the liquid fraction is formed by a liquid nitrogen stream (58) from the high-pressure column (20) or from the high-pressure column top condenser (23; 323) or a first liquid fraction is formed by a liquid oxygen stream (78) from the low-pressure column (21) or from the low-pressure column reboiler (24, 424, 488, 524) and a second liquid fraction is formed by a liquid nitrogen stream (58) from the high-pressure column (20) or from the high-pressure column top condenser (23; 323).
 6. The process as claimed in claim 4, characterized in that the first substream (37) is expanded to perform work downstream of the main heat exchanger (26) in a liquid expander (38, 938).
 7. The process as claimed in claim 1, characterized in that a fourth substream (9) of the feed air compressed to the first pressure is expanded to perform work in a second expansion machine (48) and then introduced (49) into the low-pressure column (21).
 8. The process as claimed in claim 1, characterized in that the liquid oxygen-enriched fraction (63, 65) which is introduced into the evaporation space of the high-pressure column top condenser (23) is formed by bottoms liquid (62) from the high-pressure column (20) and/or from the remaining liquid 86 from the evaporation space of the argon top condenser
 25. 9. The process as claimed in claim 1, characterized in that the liquid oxygen-enriched fraction which is introduced into the evaporation space of the high-pressure column top condenser (323) is formed by an intermediate liquid from the low-pressure column (21).
 10. The process as claimed in claim 1, characterized in that the low-pressure column (21) has an intermediate evaporator which takes the form of a condenser-evaporator, and at least a portion of the second substream (489) is introduced into the liquefaction space of the intermediate evaporator (488) downstream of the low-pressure column reboiler (424) and upstream of the argon top condenser (25).
 11. The process as claimed in claim 1, characterized in that the evaporation space of the low-pressure column reboiler (524) takes the form of a dephlegmator.
 12. The process as claimed in claim 1, referring back to claim 2, characterized in that the expansion machine is formed by two series-connected turbines (617 a, 617 b) and especially the first substream (618, 616) is introduced between the two turbines (617 a, 617 b) into the main heat exchanger (26), where it is cooled or warmed.
 13. The process as claimed in claim 12, characterized in that the two turbines (617 a, 617 b) are decelerated by one of the following methods: one recompressor (613 a, 613 b) each, where the two recompressors are connected in series, a common recompressor (813) mechanically coupled to both turbines (617 a, 617 b), one electrical generator each.
 14. The process as claimed in claim 6, characterized in that the first substream (37), downstream of the main heat exchanger (26) and upstream of the liquid expander (938), is cooled in a countercurrent subcooler (42).
 15. An apparatus for production of pure nitrogen and oxygen by cryogenic fractionation of air, comprising a distillation column system having a high-pressure column (20) and a low-pressure column (21) and also a high-pressure column top condenser (23) and a low-pressure column reboiler (24, 424, 488, 524), both of which take the form of condenser-evaporators, a main air compressor (1) for compression of the entirety of the feed air to a first pressure, a main heat exchanger (26) for cooling of feed air, means of supplying (18; 249) a first substream (10, 6, 15, 16; 209, 246, 247), cooled in the main heat exchanger (26), of the air compressed to the first pressure to the distillation column system, means of supplying a second substream (7), cooled in the main heat exchanger (26), of the air compressed to the first pressure into the liquefaction space of the low-pressure column reboiler (24, 424, 488, 524), means of introducing the second substream (31, 33, 19) liquefied liquefaction space of the low-pressure column reboiler (24, 424, 488, 524) into the distillation column system and comprising means of introducing (63, 64, 65) a liquid oxygen-enriched fraction (62, 86) into the evaporation space of the high-pressure column top condenser (23), wherein the distillation column system also includes an argon column (22) with an argon top condenser (25) and means of introducing an argon-containing oxygen stream (27) from an intermediate point in the low-pressure column (21) into the argon column (22), characterized by means of introducing the second substream (31) downstream of the low-pressure column reboiler (24, 424, 488, 524) into the evaporation space of the argon top condenser (25) and means of introducing the second substream (33) downstream of the argon top condenser (25) into the high-pressure column (20) and/or into the low-pressure column (21). 